In order to ensure that two-stroke engines have a high power capacity at high speeds, a high volumetric efficiency is required and the charge losses must be minimized. This can be accomplished by an early and therefore higher opening of the exhaust passage into the cylinder. The adjustment of the exhaust port, to obtain maximum power capacity of the engine at high speeds involves, in the medium speed range, not only an appreciable decrease of the useful stroke, but also a large increase of the charge losses. As a result, the torque decreases and the specific fuel consumption increases greatly. A higher torque in conjunction with lower fuel consumption can be obtained, at lower engine speeds, only if the opening of the exhaust port happens later in the down stroke of the piston which means that the exhaust port must be at a lower position than it is at high engine speeds.
For this purpose it is known to provide a valve in the exhaust port which is movable between a full flow position and a flow restricting position. When in the flow restricting position, the end of the valve is substantially flush with the peripheral surface of the cylinder bore. In this flow restricting position, the exhaust port is effectively lowered in relation to the down stroke of the piston. The valve is adjustable to vary the relative height of the exhaust port as is required by the given operating conditions of the engine.
U.S. Pat. No. 7,484,482 B1, issued on Feb. 3, 2009 to Mayringer, entitled “Valve Assembly for a Two-Stroke Engine”, discloses a valve assembly having a two-part valve provided, in part, in a main exhaust port and auxiliary valves provided, in part, in auxiliary exhaust ports. The valve assembly also has a valve actuator to which the valves are connected. The valve actuator is movable between a lowered position, an intermediate position and a raised position. The two-part valve has a first valve part connected to the actuator and movable with the actuator between the lowered position, the intermediate position, and the raised position. The auxiliary valves are connected to the first valve part and are movable with the first valve part and the actuator between the lowered position, the intermediate position, and the raised position. The two-part valve has a second valve part that is movable between a lowered position and a raised position. When the first valve part is in its lowered position or its intermediate position, the second valve part is in its lowered position. When the first valve part is in its raised position, the second valve part is in its raised position.
The valve actuator of Mayringer has a pressure chamber including a diaphragm. To move the valve actuator between its lowered and raised positions, different pressures are supplied to the pressure chamber from the crankcase of the engine to which the exhaust valve assembly is connected. To move the valve actuator to the raised position, positive pressure is supplied to the pressure chamber. To move the valve actuator to the lowered position, negative pressure is supplied to the pressure chamber. To move the valve actuator to the intermediate position, ambient pressure is supplied to the pressure chamber. A spring also biases the diaphragm toward a position corresponding to the intermediate position of the valve actuator.
Although the above valve actuator can move the valve actuator to three different positions, it has certain drawbacks. First, since the pressure chamber moves between its different positions as a result of a difference between the pressure inside the pressure chamber and the ambient pressure, this pressure difference can be relatively small, which yields a relatively small force to overcome the bias of the spring. Therefore this results in a relatively slow actuation time. The force can be increased by increasing the size of the diaphragm, but there is a limit to the possible size of the diaphragm.
Also, the valve actuator of Mayringer cannot provide three distinct positions in a turbocharged engine. This is because in turbocharged engines, the pressure in the crankcase is always greater than ambient. As a result, the crankcase of a turbocharged engine cannot supply a negative pressure to the pressure chamber of the valve actuator. Therefore, the valve actuator of Mayringer cannot be moved to its lowered position in a turbocharged engine.
Therefore, there is a need for an exhaust valve assembly having a valve actuator that has a relatively fast actuation and that can be used in turbocharged engines.